The present invention relates to broadcast transmission systems and is particularly directed to compensation of distortion within a digital transmission system, such as a digital TV (xe2x80x9cDTVxe2x80x9d) transmission system.
A broadcast transmission system, such as a DTV broadcast system, includes an amplifying device that increases the power of an electrical information signal such that an antenna is excited to emit a broadcast signal at a desired strength. The amplifying device is referred to as a power amplifier. In order to optimize the quality of the broadcast signal, the electrical signal is conditioned prior to amplification. The signal conditioning includes band-pass filtering the electrical signal to limit the frequency band of the electrical signal that is input to the power amplifier.
Several issues arise during operation of such a transmission system. One issue is that the components of the transmission system, including the power amplifier and the signal conditioning devices, distort the electrical information signal away from intended values. Specifically, the power amplifier imposes non-linear distortion upon the signal. Also, some of the signal conditioning devices (e.g., band-limiting filter) impose linear distortions upon the information signal.
As a result of such distortions within the transmission system, instantaneous amplitude variations (AM/AM) and instantaneous phase variations (AM/PM) occur. In addition, frequency dependent amplitude and phase variations also occur. It is to be appreciated that within a phase-amplitude modulated system, amplitude and phase integrity of the system must be preserved for optimum system performance.
Traditional equalization for television systems has been accomplished by analog, pre-distortion equalizers and correctors that are static (non-adaptive). Such equalizers and correctors require factory adjustments to provide a desired amount of pre-distortion (pre-equalization). Aging of components, and temperature change cause drift in the proper amount of pre-distortion that should be imposed by the equalizers and correctors. Occasional field adjustments are required.
Digital signal processing techniques provide improved performance of the pre-distortion of the information signal. Specifically, digital signal processing can be used in an adaptive correction/equalization approach. Such an adaptive approach can eliminate the factory and field adjustments.
It is known to perform adaptive correction of a signal within a signal stream proceeding toward an antenna. However, in a relatively fast data system, the correction requires a relatively large amount of processing in a short period of time. In one known technique, all of the distortion (i.e., linear and non-linear) is corrected in a single step.
In another technique, the correction for the distortion imposed within the system is done component by component proceeding in a direction toward the antenna. Specifically, for each component, the signal that is output from that component is monitored to determine the amount of distortion imposed by that component. A correction is then developed for that component. Subsequently, the next component along the signal stream is monitored to develop the correction for that component. However, such a technique is time consuming and is often unsuitable for a high data rate stream. Thus, there is a need for a high-speed technique for adaptive correction of linear and non-linear distortion within a digital broadcast transmission system.
A second issue that presents itself is that the power amplifier may impose a frequency spectrum spread on the signal during amplification. The spreading may include smearing of the frequency and generation of unwanted frequency components. The frequency spread results in a broadcast signal of diminished quality. Additional signal conditioning, primarily in the form of band-pass filtering, after amplification will improve the quality of the broadcast signal. However, each additional signal-conditioning component (e.g., a band-pass filter) causes additional distortions to the signal. An increase in the number of distortion-causing components within the system is associated with an increase in the distortions that must be corrected. Current microprocessors are not able to provide the distortion correction and correction adaptation needed for high data-rate signals conveyed within a system that has a relatively large number of distortion-causing components. Further, within such a system, amplitude or group delay variations over frequency will reduce the effectiveness of any instantaneous non-linear correction applied. Thus, there is a need for a technique for distortion correction and correction adaptation for a system that has a relatively large number of distortion-causing components.
The present invention provides a broadcast transmission system that includes means for providing an information signal at a first rate value. Interpolation means increases the rate from the first value to a second value. Power amplifier means amplifies the information signal to a broadcast transmission power level. The power amplifier means is located downstream of the interpolation means. Post-amp conditioning means conditions the information signal after the information signal is amplified by the power amplifier means. The post-amp conditioning means is located downstream of the power amplifier means, and subjects the information signal to distortion shifts away from intended values. Pre-equalizer means modifies the information signal while the signal is at the first rate to compensate for the distortion shifts imposed by the post-amp conditioning means. The pre-equalizer means is located upstream of the interpolation means.